The present invention generally relates to modulated oscillators and more specifically relates to an angle modulatable radio frequency oscillator in which the element or elements impressing the modulation upon the radio frequency energy are a significant factor in the determination of the oscillator center frequency. Such elements, because of their close coupling to the oscillator, may substantially determine the temperature induced changes in oscillator center frequency and may therefore be employed in the frequency stabilization of the oscillator center frequency without detriment to the linearity of modulation.
The radio frequency (RF) generator in most radio transmitters is an electronic circuit which, with appropriate feedback, oscillates at a preselected frequency. Information may be impressed upon this preselected frequency, or center frequency, in several ways. One common way is to angle modulate the oscillator center frequency thereby frequency or phase modulating information on the carrier of the radio frequency.
Low frequency RF oscillators, which have their center frequency determined by a piezoelectric device, have been shown capable of being angled modulated with a voltage variable capacitor diode, commonly known as a varactor. One such oscillator is shown in U.S. Pat. No. 3,528,032, assigned to the assignee of the present invention. There, sensitivity of the modulation element to a modulation signal is essentially determined by the amount of reactance change which can be achieved by the varactor. Because of the large reactance values found in a crystal oscillator, the amount of modulation which can be achieved by a varactor diode is small, due to its relatively small reactance value. Means for controlling the amount of sensitivity has been described in U.S. Pat. Nos. 3,916,344 and 4,378,534, each assigned to the assignee of the present invention.
At microwave frequencies, generally considered to be frequencies above 1,000 MHz, it is common to oscillate at relative high power levels by including an active device such as a Gunn diode or transistor in a feedback network comprised of a high Q electronically resonant structure which determines the frequency of oscillation by its electronic dimensions. Oscillators of this type are described in U.S. Pat. No. 4,270,098 and U.S. Pat. No. 4,542,352, Yester, Jr. et al. each assigned to the assignee of the present invention.
The transmitters employed in the radio communications services at microwave frequencies are generally required to modulate the center frequency with a wide bandwidth modulating signal. This modulating signal may consist of a large number of independent narrow band channels, such as an audio channel of 4 KHz, multiplexed together into a baseband signal of a bandwidth of 2 MHz or more in bandwidth. Methods employed previously to modulate the oscillator center frequency have employed a separate resonant network including a varactor which is loosely coupled to the main frequency determining network, or tank, of the oscillator. (See U.S. Pat. No. 4,542,352, Yester, Jr., et al., and U.S. Pat. Nos. 4,270,098; 3,747,032; and 3,601,723). These multiple resonant tank approaches are typically most applicable to frequency modulated applications where octave or more of operational bandwidth is not expected. Previous realizations of multiple resonant tank approaches have generally selected one of the following methods. First, two resonant tanks are coupled via an impedance inverter to realize modulation with a linear change in frequency with applied modulation voltage. Second, a resonator containing a varactor is coupled to the frequency determining tank via a probe having a unique physical configuration (U.S. Pat. Nos. 3,747,032 and 4,375,621). Third, a resonant network is tuned to a harmonic of the center frequency reportedly effecting a broader frequency versus tuning curve (U.S. Pat. No. 3,982,211). And fourth, employing either a varactor tuned shunt circuit added to the resonant active circuit or a paralleled fixed-tuned resonator circuit added to a varactor tuned circuit.
Analysis of dual tank impedance inverter coupled circuits shows that an oscillator employing such a circuit for introducing modulation has a linear frequency vs. voltage characteristic when the varactor parasitic shunt capacitance is resonant with the inductance of coupling mechanism and the varactor normalized capacitance tuning curve is indicative of a normalized capacitance vs. normalized applied voltage relationship having a "gamma" equal to 1. The capacitance to voltage relationship may be expressed as: EQU C=Co/(1+V/.phi.).sup..gamma.
where:
C=varactor junction capacitance PA1 V=applied voltage PA1 .phi.=semiconductor contact potential PA1 Co,.gamma.=constants PA1 C'=C/Co PA1 V'=1+V/.phi.
and normalized to: EQU C'=(V').sup.-.gamma.
where:
Thus, the dual tank impedance inverter coupled circuit requires the approximation of a perfectly linear circuit by employing a varactor having a normalized capacitance equal to an inverse normalized applied voltage raised to a constant power (gamma) which is equal to 1 (gamma=1). As a practical matter, gamma is a function of applied voltage and somewhat difficult to control and maintain. (See Androski, Lentz, and Salvage, 4A FM Transmitter and Receiver, Bell System Technical Journal, Vol. 50 No. 7, September 1971, pg. 2255-58).
Linearity of modulation for complex wide bandwidth modulating signals is important for prevention of intermodulation distortion in the transmitted signal. In a microwave radio system transmitting a large number of frequency division multiplex (FDM) channels, for example 600 channels, excessive intermodulation would result in cross talk and excessive noise between the FDM channels. Ideally, for no intermodulation distortion in a frequency modulated oscillator, the frequency versus modulation voltage curve should be perfectly linear; that is, the second derivative, d.sup.2 F/dV.sup.2, and all higher order derivatives should be equal to zero. However, since second order differential-gain slope variations (d.sup.2 F/dV.sup.2 =non zero constant) occur in other parts of the radio transmission and reception system, a perfect straight-line frequency vs. modulation tuning curve at the transmitter modulated oscillator may not be desired. Thus, it may be desirable to have a variable amount of second derivative in the frequency versus modulation voltage curve in the transmitter modulated oscillator with a "linearity" adjustment to effect an adjustment to overall zero and non-zero values of second derivative. Higher order derivatives of the frequency versus modulation voltage curve are highly undesirable and are conventionally minimized by varying the coupling of the modulation tank to the frequency determining tank of the modulating oscillator until the amount of higher order derivatives are minimized for a particular center frequency.
If the linearity adjustment, selecting a compensating second derivative value (or zero) and minimizing higher order derivatives, can be made independent of the modulation voltage and bias point of the varactor diode, the same varactor diode used for modulation may be employed to control the center frequency of the modulated oscillator. Such a control would enable the oscillator center frequency to be maintained at a preselected frequency despite changes in the resonant point of the frequency determining elements over temperature. However, tight coupling of the varactor to the frequency determining elements of the oscillator feedback circuitry to accomplish these features has been contraindicated by several authorities. Degradation of oscillator noise performance and temperature stability performance due to the limited Q of the varactor have been projected penalties.